(a) Field of the Invention
The present invention relates to a positive active material composition for a rechargeable lithium battery and a method of preparing a positive electrode using the same and, more particularly, to a positive active material composition for a rechargeable lithium battery exhibiting long cycle life characteristics.
(b) Description of the Related Art
Rechargeable lithium batteries employ materials into or from which lithium ions are intercalated or deintercalated as negative and positive active materials.
Metallic lithium has been used as the negative active material in rechargeable lithium batteries. However, lithium metal has good reactivity toward electrolyte and deposits to form a dendric layer which induces short circuits between the negative and positive active material. There are at least two disadvantageous effects that arise from the reaction of lithium with electrolyte. One is the exothermic liberation of heat and the other is the formation of passive films on lithium, which has been shown to be one of the reasons for the loss of capacity of lithium cells upon repeated cycling.
The problem of lithium reactivity toward the electrolyte is addressed by replacing lithium metal with carbon-based materials. With the use of carbon-based active materials, the potential safety problem present in metallic lithium-based batteries can be prevented while achieving a relatively higher energy density as well as a reasonable shelf life.
For the positive active material in the rechargeable lithium battery, chalcogenide compounds into or from which lithium ions are inserted or extracted are used. Typical examples of the positive active material include LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (0≦x≦1) or LiMnO2. LiCoO2 is widely used as it has an electrical conductivity of about 10−2 to 1 S/cm at room temperature and high battery voltage, but has poor safety characteristics during high rate charges and discharges. LiNiO2 has a high charge capacity, but is difficult to produce. Manganese-based material such as LiMn2O4 or LiMnO2, etc. is easiest to prepare, is less expensive than the other materials, and has environmentally friendly characteristics. Such manganese-based compounds have low capacities, but LiMn2O4 is particularly stable for the battery use and thus attractive for electric vehicle application.
However, when a battery using manganese-based materials such as LiMn2O4 is charged and discharged for a long time, particularly, at a high temperature, a side reaction between the electrolyte and the manganese-based material occurs at the surface of the manganese-based material. It is believed that H2O reacts with LiPF6 in the electrolyte to generate strong acid such as hydrogen fluoride (HF) which attacks the Mn present on the surface of the manganese-based active material and the attacked Mn is eluted into the electrolyte. The eluted Mn is dissolved in the electrolyte and the active material disintegrates. This side reaction seriously deteriorates the cycle life characteristics of the battery.
An attempt to solve this problem has been made by synthesizing a material including 1 or more equivalent Li or by partially substituting oxygen with F in a spinel manganese-based material. However, these methods do not effectively improve the cycle life characteristics of the battery, particularly, at high temperatures.
Furthermore, such problem due to the reaction between the positive active material and the electrolyte may occur in manganese-based active material as well as in cobalt- and nickel-based active materials. Therefore, the studies to reduce the reaction between the positive active material and the electrolyte are attempted, but the satisfied effect is not obtained.